The person who is being cheated on in a relationship. The cheater is the person who engages in an affair with a person who is in a relationship and the cheatee is the person who is cheated on. Taken from the word *employee*, you can add ee to many words to define them as the person who's directly involved in that situation. See the tags for similar examples.
Did you hear? Alex is cheating on his girlfriend Tina! Man she's gonna be pissed off when she finds out she's the cheatee!